(1) Field of the Invention
This invention relates to a non-aqueous cell comprising, in a cell case, a positive electrode having a positive electrode active material composed of lithium cobalt oxide, a negative electrode, and an electrolyte.
(2) Description of the Prior Art
In recent years, lithium ion cells have attracted attention as high capacity batteries. In such lithium ion cells, lithium cobalt oxides and lithium manganese oxides are used as a material for the positive electrode, and alloys or carbon materials are used for a negative electrode active material thereof Such lithium ion cells, however, have such drawbacks that, as charge-discharge cycles are repeated, a charge and discharge capacity and a charge and discharge efficiency are degraded.
Japanese Patent Publication No. 63-59507, for example, discloses a non-aqueous electrolyte cell in which LixMyO2 (xe2x80x98Mxe2x80x99 consists of Ni or Co, x less than 0.8, and y≈1) is used as a positive electrode active material and a lithium metal is used for the negative electrode, and this cell exhibits a high electromotive force of 4 V or higher and a high energy density. However, this cell as well has such drawbacks that a charge and discharge capacity is susceptible to degrading, and so forth. The reasons for such drawbacks seem to be that an irreversible change occurs in part of the crystal structure of the positive electrode active material, causing deterioration in the capability of absorbing and releasing lithium ions, and that decomposition of the electrolytic solution or the like is caused by overcharge and overdischarge that exceed an appropriate electric potential range, inducing a shortage of the electrolytic solution, undesirable effects by the decomposed matter, and so forth. In view of this drawback, a technique to replace part of the crystal of the positive electrode active material with other types of metallic elements has been suggested as a means to prevent such undesirable phenomena from occurring.
For example, Japanese Unexamined Patent Publication Nos. 4-329267 and 5-13082 disclose a battery using a positive electrode active material in which a titanium compound is added to a lithium cobalt oxide in a state of solid solution.
In addition, Japanese Unexamined Patent Publication No. 4-319260 discloses a battery using a positive electrode active material in which a zirconium is added to a lithium cobalt oxide in a state of solid solution.
Also, Japanese Unexamined Patent Publication No. 4-253162 discloses a battery using a positive electrode active material in which a one element selected from lead, bismuth, and boron is added to a lithium cobalt oxide in a state of solid solution.
The techniques as listed above can improve the problems as described above, but on the other hand, they induce other drawbacks, such that an initial capacity of the cell is reduced, and that a temperature at which the positive electrode active material starts to generate heat is lowered and thereby safety of the cell is degraded.
As portable appliances such as portable computers and mobile phones have increasingly become popular in the market, a need for a cell having an excellent low-temperature cycle characteristic and increased safety is accordingly growing. In addition, in order to further reduce sizes and weight of batteries, a thin type sealed cell in which a power-generating component is enclosed in a cell case composed of a flexible and lightweight laminated material (such a cell case is hereinafter also referred to as a xe2x80x9claminated containerxe2x80x9d) has been developed. However, such a laminated container has a small strength against a cell internal pressure, and therefore easily expands when an internal gas is formed in the cell, causing problems such as deformation of the cell, leakage of the electrolyte, and rupture of the cell. For this reason, particularly in such a thin type cell, a positive electrode active material that can reduce an internal gas formation is desired.
In view of the foregoing problems and drawbacks in prior art, it is an object of the present invention to provide a non-aqueous electrolyte cell having a high working voltage, excellent low-temperature discharge characteristics, a reduced internal gas generation, and an increased safety.
This and other objects are accomplished in accordance with the present invention, by providing a non-aqueous electrolyte cell comprising in a cell case a positive electrode having a positive electrode active material composed of a lithium cobalt oxide, a negative electrode, and an electrolyte comprising a non-aqueous solvent,
the positive electrode active material comprising a Ti-attached LiCoO2 in which a particle of a titanium and/or a titanium compound is attached on a surface of a particle of the lithium cobalt oxide.
In the Ti-attached LiCoO2 having such a configuration as described above, titanium particles and/or titanium compound particles are attached on at least a surface of lithium cobalt oxide, and the titanium particles and/or titanium compound particles serve to decompose a film derived from the non-aqueous solvent (the film is formed such as to surround the positive electrode active material), or to facilitate the exfoliation of the formed film. Therefore, according to the configuration as described above, degradation of discharge performance caused by a poor ionic conductivity can be suppressed, and consequently a significant decrease in discharge capacity under low temperature can be avoided. Note here that in the case of prior art positive electrode active materials, a surface of the positive electrode active material is surrounded by a film derived from a non-aqueous solvent, inhibiting the contact between the active material particles and the electrolyte. Therefore, in the case of prior art positive electrode active materials, the discharge performance is significantly degraded under a low temperature environment of 0xc2x0 C. or lower, where the ionic conductivity is low.
Moreover, according to the configuration as described above, not only do the titanium particles and/or titanium compound particles, which are present on the surface of the lithium cobalt oxide, serve to facilitate the charge-discharge reaction, but also serve to inhibit decomposition of the electrolytic solution, thereby suppressing formation of internal gas.
Furthermore, the Ti-attached LiCoO2 has a higher temperature point at which heat is generated, when compared to titanium-lithium cobalt oxide in a form of solid solution. Therefore, by employing the Ti-attached LiCoO2, it is made possible to provide cell having an improved safety compared to the case where the titanium-lithium cobalt oxide in a form of solid solution is employed.
In the above-described configuration of the non-aqueous electrolyte cell, it is preferable that a mole ratio in the Ti-attached LiCoO2 of the titanium and/or the titanium compound to the lithium cobalt oxide be in the range of from 0.00001 to 0.02, and more preferably be in the range of from 0.00001 to 0.01.
When the mole ratio x is restricted to be 0.00001xe2x89xa6xxe2x89xa60.02, a working voltage and a discharge capacity at a low temperature of 0xc2x0 C. or below can be remarkably increased without substantially decreasing a discharge capacity at room temperature. Moreover, when the mole ratio (x) is restricted to be 0.00001xe2x89xa6xxe2x89xa60.01, the decrease of the discharge capacity at room temperature can be further lessened, and thus the low-temperature discharge characteristics are further improved. It is noted here that if the mole ratio x is less than 0.00001, the low-temperature discharge characteristics cannot be sufficiently improved. If the mole ratio x is more than 0.02, no further improvement in the low-temperature discharge characteristics is observed, and the discharge characteristics under room temperature are deteriorated. Hence, the mole ratio x is preferable to be 0.00001xe2x89xa6xxe2x89xa60.02, and more preferably be 0.00001xe2x89xa6xxe2x89xa60.01.
Further, in the above-described configuration of the non-aqueous electrolyte cell, the titanium compound may be titanium oxide and/or lithium-titanium complex oxide. Such compounds as well achieve the same effects as attained by the titanium compound. An example of the lithium-titanium complex oxide includes a lithium titanium oxide.
Further, in the above-described configuration of the non-aqueous electrolyte cell, the electrolyte may contain a mixed solvent composed mainly of ethylene carbonate.
Ethylene carbonate has a high permittivity and therefore is an ideal non-aqueous solvent. In addition, a film derived from the ethylene carbonate is easily decomposed by the catalytic action by titanium, and therefore it is unlikely to result in such a film that inhibits the contact between the positive electrode active material and the electrolyte. Hence, when a cell uses an electrolyte comprising a mixed solvent composed mainly of ethylene carbonate, transfer of ions at the interface of the positive electrode active material is performed smoothly, thus effectively preventing the discharge performance in a low temperature range from deteriorating.
Further, in the above-described configuration of the non-aqueous electrolyte cell, the electrolyte may contain an imide salt represented by the structural formula LiN(SO2C2F5)2.
LiN(SO2C2F5)2 does not generates much hydrogen fluoride (HF), and therefore, the effect of the side reaction derived from LiN(SO2C2F5)2 is little, thus making it possible to reduce the amount of generated gas under a state of a high electric potential or a high temperature.
In the above-described configuration of the non-aqueous electrolyte cell, the electrolyte may comprise a mixed solvent containing ethylene carbonate and diethyl carbonate.
Ethylene carbonate and diethyl carbonate exhibit a high degree of chemical stability. Therefore, by employing a mixed solvent containing these solvents as an electrolyte component, the amount of generated gas can be reduced. In particular, when the electrolyte comprises the mixed solvent containing these solvents and, as an electrolyte salt, LiN(SO2C2F5)2, the amount of generated gas can be remarkably reduced.
In the above-described configuration of the non-aqueous electrolyte cell, the electrolyte may be a gel type solid polymer electrolyte.
Generally, ionic conductivity at the interface of the positive electrode active material declines as a temperature goes down, and this tendency becomes more apparent in the case of gel type electrolytes. However, as previously mentioned, when a cell employs the Ti-attached LiCoO2 in which titanium oxide or the like is added to lithium cobalt oxide, titanium oxide or the like serves to facilitate the transfer of ions at the interface of the active material particles, thereby preventing a significant decrease in the discharge capacity in a low temperature range. Hence, the advantageous effects by employing the Ti-attached LiCoO2 as a positive electrode active material are more apparent in the case of a cell employing a gel type solid polymer electrolyte.
In the above-described configuration of the non-aqueous electrolyte cell, the cell case may be composed of a laminated material in which an aluminum film and a resin film are laminated.
Since such a laminated material is lightweight and flexible, thin and lightweight sealed cells can be efficiently manufactured by employing a cell case composed of a laminated material. However, such a cell case has such drawbacks that such a case can be easily deformed in the case where a cell internal pressure is generated, resulting in leakage of the electrolyte and rupture of the cell. By using the Ti-attached LiCoO2 as a positive electrode active material, such drawbacks of the cell case composed of a laminated material can be overcome. In the case of the cell case composed of a laminated material, it is more preferable that the cell employs Ti-attached LiCoO2 as a positive electrode active material, and an electrolyte comprising a mixed solvent containing LiN(SO2C2F5)2 and ethylene carbonate. With this configuration, as has been described above, the amount of generated internal gas is remarkably reduced, and therefore, the rupture of the cell or the leakage of the electrolyte does not occur easily even when the cell case composed of a laminated material is employed. In other words, this configuration achieves a thin type sealed cell having excellent reliability and safety.
In addition, in all the above-described configurations of the non-aqueous electrolyte cell, the negative electrode active material may be a graphite capable of absorbing and releasing lithium ions.
The negative electrode employing a graphite as an active material can achieve a larger discharge capacity than a negative electrode using coke can, particularly under low-temperature.
The non-aqueous electrolyte cell according to the present invention can be produced by a method of producing a non-aqueous electrolyte cell comprising in a cell case a positive electrode having a positive electrode active material composed of a lithium cobalt oxide, a negative electrode, and an electrolyte comprising a, non-aqueous solvent, the method comprising the step of:
producing a positive electrode active material by preparing a Ti-attached lithium cobalt oxide in which a titanium oxide particle and/or a metallic titanium particle is attached on a surface of a lithium cobalt oxide particle, such that a lithium cobalt oxide powder is mixed with a titanium oxide powder and/or a metallic titanium powder, and thereafter the mixture is baked.
It is to be noted here that the Ti-attached lithium cobalt oxide of the present invention is intended to represent such a substance that a titanium particle and/or a metallic titanium particle is attached to or bonded with the surface of a lithium cobalt oxide particle, which is a main substance of the positive electrode active material. According to the above-described method, such a Ti-attached lithium cobalt oxide is produced with a high productivity.
In the above-described method, a lithium-titanium complex oxide may be used in place of the titanium oxide powder and/or the metallic titanium powder.
In addition, the above-described producing method may be such a method as in the following. Specifically, the step of preparing a positive electrode active material may comprise the steps of:
preparing a titanium-compound-mixed cobalt oxide by baking a cobalt oxide powder and a titanium oxide powder and/or a metallic titanium powder,
crushing and grinding the titanium-compound-mixed cobalt oxide into a powdered state,
preparing a Ti-attached lithium cobalt oxide in which a particle of titanium oxide and/or metallic titanium is attached on a surface of each lithium cobalt oxide particle, by mixing the powdered titanium-compound-mixed cobalt oxide with at least one lithium compound selected from the group consisting of lithium hydroxide, lithium carbonate, and lithium nitrate, and then baking the mixture.
According to such a method comprising these steps, Ti-attached LiCoO2 can be produced with a low cost.